Slide 1 Chapter 9 Strings

Slide 2 Learning Objectives An Array Type for Strings C-Strings Character Manipulation Tools Character I/O get, put member functions putback, peek, ignore Standard Class string String processing

Slide 3 Introduction Two string types: C-strings Array with base type char End of string marked with null, ‘\0’ ‘Older’ method inherited from C String class Uses templates

Slide 4 C-Strings Array with base type char One character per indexed variable One extra character:  ‘\0’ Called ‘null character’ End marker We’ve used c-strings Literal “Hello” stored as c-string

Slide 5 C-String Variable Array of characters: char s[10]; Declares a c-string variable to hold up to 9 characters + one null character Typically ‘partially-filled’ array Declare large enough to hold max-size string Indicate end with null Only difference from standard array: Must contain null character

Slide 6 C-String Storage A standard array: char s[10]; If s contains string “Hi Mom”, stored as: Display page 352

Slide 7 C-String Initialization Can initialize c-string: char myMessage[20] = “Hi there.”; Needn’t fill entire array Initialization places ‘\0’ at end Can omit array-size: char shortString[] = “abc”; Automatically makes size one more than length of quoted string NOT same as: char shortString[] = {‘a’, ‘b’, ‘c’};

Slide 8 C-String Indexes A c-string IS an array Can access indexed variables of: char ourString[5] = “Hi”; ourString[0] is ‘H’ ourString[1] is ‘i’ ourString[2] is ‘\0’ ourString[3] is unknown ourString[4] is unknown

Slide 9 C-String Index Manipulation Can manipulate indexed variables char happyString[7] = “DoBeDo”; happyString[6] = ‘Z’; Be careful! Here, ‘\0’ (null) was overwritten by a ‘Z’! If null overwritten, c-string no longer ‘acts’ like c-string! Unpredictable results!

Slide 10 Library Declaring c-strings Requires no C++ library Built into standard C++ Manipulations Require library <cstring> Typically included when using c-strings Normally want to do ‘fun’ things with them

Slide 11 = and == with C-strings C-strings not like other variables Cannot assign or compare: char aString[10]; aString = “Hello”; // ILLEGAL! Can ONLY use ‘=‘ at declaration of c-string! Must use library function for assignment: strcpy(aString, “Hello”); Built-in function (in <cstring>) Sets value of aString equal to “Hello” NO checks for size! Up to programmer, just like other arrays!

Slide 12 Comparing C-strings Also cannot use operator == char aString[10] = “Hello”; char anotherString[10] = “Goodbye”; aString == anotherString;   // NOT allowed! Must use library function again: if (strcmp(aString, anotherString)) cout << “Strings NOT same.”; else cout << “Strings are same.”;

Slide 13 The <cstring> Library Full of string manipulation functions Display 9.1, page 357

Slide 14 The <cstring> Library Cont’d Full of string manipulation functions Display 9.1, page 357

Slide 15 C-string Functions:  strlen() ‘String length’ Often useful to know string length: char myString[10] = “dobedo”; cout << strlen(myString); Returns number of characters Not including null Result here: 6

Slide 16 C-string Functions:  strcat() strcat() ‘String concatenate’: char stringVar[20] = “The rain”; strcat(stringVar, “in Spain”); Note result: stringVar now contains “The rainin Spain” Be careful! Incorporate spaces as needed!

Slide 17 C-string Arguments and Parameters Recall:  c-string is array So c-string parameter is array parameter C-strings passed to functions can be changed by receiving function! Like all arrays, typical to send size as well Function ‘could’ also use ‘\0’ to find end So size not necessary if function won’t change c-string parameter Use ‘const’ modifier to protect c-string arguments

Slide 18 C-String Output Can output with insertion operator, << As we’ve been doing already: cout << news << “ Wow.\n”; Where news is a c-string variable Possible because << operator is overloaded for c-strings!

Slide 19 C-String Input Can input with extraction operator, >> Issues exist, however Whitespace is ‘delimiter’ Tab, space, line breaks are ‘skipped’ Input reading ‘stops’ at delimiter Watch size of c-string Must be large enough to hold entered string! C++ gives no warnings of such issues!

Slide 20 C-String Input Example char a[80], b[80]; cout << “Enter input:  “; cin >> a >> b; cout << a << b << “END OF OUTPUT\n”; Dialogue offered: Enter input:  Do be do to you! DobeEND OF OUTPUT Note:  Underlined portion typed at keyboard C-string a receives:  “do” C-string b receives:  “be”

Slide 21 C-String Line Input Can receive entire line into c-string Use getline(), a predefined member function: char a[80]; cout << “Enter input: “; cin.getline(a, 80); cout << a << “END OF OUTPUT\n”; Dialogue: Enter input:  Do be do to you! Do be do to you!END OF INPUT

Slide 22 More getline() Can explicitly tell length to receive: char shortString[5]; cout << “Enter input: “; cin.getline(shortString, 5); cout << shortString << “END OF OUTPUT\n”; Results: Enter input:  dobedowap dobeEND OF OUTPUT Forces FOUR characters only be read Recall need for null character!

Slide 23 Character I/O Input and output data ALL treated as character data e.g.:  number 10 outputted as ‘1’ and ‘0’ Conversion done automatically Uses low-level utilities Can use same low-level utilities ourselves as well

Slide 24 Member Function get() Reads one char at a time Member function of cin object: char nextSymbol; cin.get(nextSymbol); Reads next char & puts in variable nextSymbol Argument must be char type Not ‘string’!

Slide 25 Member Function put() Outputs one character at a time Member function of cout object: Examples: cout.put(‘a’); Outputs letter ‘a’ to screen char myString[10] = “Hello”; cout.put(myString[1]); Outputs letter ‘e’ to screen

Slide 26 More Member Functions putback() Once read, might need to ‘put back’ cin.putback(lastChar); peek() Returns next char, but leaves it there peekChar = cin.peek(); ignore() Skip input, up to designated character cin.ignore(1000, ‘\n’); Skips at most 1000 characters until ‘\n’

Slide 27 Character-Manipulating Functions Display 9.3, page 372-373

Slide 28 Character-Manipulating Functions Cont’d Display 9.3, page 372-373

Slide 29 Standard Class string Defined in library: #include <string> using namespace std; String variables and expressions Treated much like simple types Can assign, compare, add: string s1, s2, s3; s3 = s1 + s2; //Concatenation s3 = “Hello Mom!” //Assignment Note c-string “Hello Mom!” automatically converted to string type!

Slide 30 Program Using Class string Display 9.4, page 377

Slide 31 I/O with Class string Just like other types! string s1, s2; cin >> s1; cin >> s2; Results: User types in: May the hair on your toes grow long and curly! Extraction still ignores whitespace: s1 receives value “May” s2 receives value “the”

Slide 32 getline() with Class string For complete lines: string line; cout << “Enter a line of input: “; getline(cin, line); cout << line << “END OF OUTPUT”; Dialogue produced: Enter a line of input: Do be do to you! Do be do to you!END OF INPUT Similar to c-string’s usage of getline()

Slide 33 Other getline() Versions Can specify ‘delimiter’ character: string line; cout << “Enter input: “; getline(cin, line, ‘?’); Receives input until ‘?’ encountered getline() actually returns reference string s1, s2; getline(cin, s1) >> s2; Results in:  (cin) >> s2;

Slide 34 Pitfall:  Mixing Input Methods Be careful mixing cin >> var and getline int n; string line; cin >> n; getline(cin, line); If input is:  42 Hello hitchhiker. Variable n set to 42 line set to empty string! cin >> n skipped leading whitespace, leaving ‘\n’ on stream for getline()!

Slide 35 Class string Processing Same operations available as c-strings And more! Over 100 members of standard string class Some member functions: .length() Returns length of string variable .at(i) Returns reference to char at position i

Slide 36 Class string Member Functions Display 9.7, page 386

Slide 37 C-string and string Object Conversions Automatic type conversions From c-string to string object: char aCString[] = “My C-string”; string stringVar; stringVar = aCstring; Perfectly legal and appropriate! aCString = stringVar; ILLEGAL! Cannot auto-convert to c-string Must use explicit conversion: strcpy(aCString, stringVar.c_str());

Slide 38 Summary C-string variable is ‘array of characters’ With addition of null character, ‘\0’ C-strings act like arrays Cannot assign, compare like simple variables Libraries <cctype> & <string> have useful manipulating functions cin.get() reads next single character getline() versions allow full line reading Class string objects are better-behaved than c-strings